Stereotactic radiosurgery (SRS) has been the treatment of choice for patients with brain tumors1-3 or functional disorders.4,5 For example, the Leksell Gamma Knife (LGK) (Elekta, Norcross, Ga.) has been used successfully to deliver a high dose to small target volumes by focusing 201 60Co sources placed in a nearly complete hemispherical arrangement to one point. For the LGK model 4C, the size of the high dose volume delivered by one shot is determined by one of four possible collimator helmets with collimators ranging in size from 4 to 18 mm in diameter. The new model of the LGK system, Perfexion™ (PFX), adopts technical innovations that include eight independent position-controlled source modules (sectors) and a stationary built-in collimator system with three apertures for each source corresponding to 4, 8, and 16 mm field sizes. Thus, a single shot may simultaneously include multiple collimator sizes. The new system eliminates the need for time consuming manual installation of collimator helmets as in the older models of LGK and moves sources in sectors over the planned aperture positions in the collimator. This new technology has led to a possible improvement in efficiency in treatment planning, delivery, and workflow.6,7 
Radiosurgery, characterized by a single high dose fraction, requires regular and careful quality assurance (QA).8 Checking the integrity of collimator systems is one of the important physics QA items required to ensure the accuracy of dose delivery. Although a simple visual inspection was sufficient for checking the geometrical configuration of the collimators in older versions of LGK, there is no easy way to independently evaluate the built-in collimator of the PFX. The conventional method of exposing a film at the isocenter provides a composite dose image, which has been successfully used for checking coincidence between the radiation and mechanical isocenters and also for dose rate verification. However, it is difficult to interpret the film in terms of the integrity of each individual source and corresponding collimator system. Maitz et al.8 used sequential strips of film around a spherical phantom with a 4 mm diameter lead ball at the center of the phantom. Images of the lead ball were identified within each exit image of the beam to verify the source-collimator alignment.
It may be useful to have a system and method of verifying the geometric configuration of the source and collimator modules of a radiosurgery system, such as a stereotactic radiosurgery system, or a radiotherapy system.